Dear Dr. Vinny,

What’s the difference between Brix, Baumé, Oechsle and residual sugar?

—Adam, Brooklyn, N.Y.

Dear Adam,

Grapes, like all fruits, contain sugar. As grapes ripen, the amount of sugar increases. Therefore, in addition to tasting grapes, measuring their sugar content is a reliable way to get an indication of ripeness, or how quickly grapes are ripening. Sugar level is important to winemakers because it not only affects the flavor of the grapes (and the wine made from them), but also the potential percent of alcohol in the final product. After all, fermentation converts the sugar in the grapes into alcohol with the help of yeast. Many winemakers will track and refer to these data points, both when deciding to pick grapes and during the fermentation process to make various decisions.

Brix, Baumé and Oechsle (as well as the Austrian system of Klosterneuburger Mostwaage, or KMW), are all measurements of sugar—kind of like taking a wine’s sugar temperature. There’s some science (and a bunch of math) that distinguishes them, typically cultural details about which measurements are used. Baumé is commonly used in Europe and Australia, Brix popular in the United States, and Oechsle is a German and Swiss favorite.

If you’re interested in the tools used to take these measurements and some of the science behind them, check out my explanation of saccharometers and refractometers.

Of course, sugar is just one of the contributors of grape and wine flavor and aroma. There are also acids and tannins and the development of the skins, seeds and stems. It’s not unusual for at least a little bit of unfermented sugar to be left at the end of fermentation, which is known as “residual sugar.” Sweeter wines will have more residual sugar than wines that are dry.

—Dr. Vinny